To interrogate, i.e. to read, passive sensor elements, an electrical test voltage is generally applied to the sensor element and the current flowing as a result of the applied voltage is measured. This allows the present electrical state of the sensor element to be calculated and permits the variable which is to be measured (pressure, temperature, etc.) to be ascertained. It should be noted that the passive sensor elements can have a variable resistance, for example, such as pressure-dependent or temperature-dependent resistances, or a variable capacitance. In this context it is clear that the test voltage comprises a DC voltage or an AC voltage, depending on the sensor element's property which is to be measured.
A sensor device having pressure-sensitive sensors can be used, by way of example, in a seat-occupation recognition system for controlling an active passenger hold-back system in a vehicle. Such a sensor mat comprises a plurality of individual pressure-sensitive sensors which are incorporated over the area of the passenger seat, distributed in the seat. The sensors are connected to an evaluation unit which checks the trigger state of the individual sensors. If the seat is being occupied by a person, a plurality of the sensors are triggered on account of the natural weight exerted on the seat by a person, a fact which is recognized by the connected evaluation circuit as being a state of occupation of the seat, and which is forwarded to the airbag controller.
To be able to interrogate the sensors selectively, each of the sensors needs to be connected to the evaluation circuit in principle. To reduce the number of connecting lines for this, it is advantageous to operate the individual sensors in a matrix configuration. This means that, with a number of n*m sensor elements, essentially n row conductors and m column conductors are provided, with one of the sensor elements being connected between a row conductor and a column conductor, respectively. It should be noted in this context that such a matrix configuration represents a circuit arrangement. That is to say that a matrix configuration in a real arrangement requires neither the sensor elements to be arranged in a regular grid distribution nor the individual connecting conductors to run rectilinearly and parallel or at right angles to one another.
To evaluate a sensor arrangement in a matrix configuration, the procedure is as follows: first, the entire matrix configuration with the exception of a first column conductor is connected to the same potential, e.g. to ground. A test voltage is now applied to the first column conductor and then the current flowing away on the individual row conductors is measured selectively. This allows the resistance values of the sensor elements connected between the first column conductor and the various row conductors to be ascertained selectively. If this procedure is repeated for each of the column conductors, all the sensor elements can be selectively tested in succession. It should be noted at this point that it is alternatively possible to apply the test voltage to the individual row conductors and to measure the current flowing away via the sensor elements on the column conductors in order to interrogate the individual sensor elements.
If such a circuit arrangement having sensor elements is used in safety-related areas, such as in the airbag controller described above in a vehicle, it is essential to be able to recognize and, if appropriate, compensate for any malfunction in the circuit arrangement on account of a fault in the conductor. In a simple matrix configuration, the result of a conductor break is, by way of example, that all the sensor elements downstream of the conductor break (seen from the evaluation circuit) can no longer be read. If the conductor break is not recognized by the evaluation circuit, then such a fault results in the conclusion that the sensor elements in question have not been triggered and therefore the actual seat-occupation situation is interpreted incorrectly. Suitable means therefore need to be provided which at least allow the occurrence of a fault to be recognized.
For this purpose, EP-A-0 895 091 proposes a circuit arrangement in which, besides the n row conductors and the m column conductors, an additional row conductor and an additional column conductor are respectively provided. In this case, the free ends of the m column conductors are connected to the additional row conductor via a respective monitoring resistor, and the free ends of the n row conductors are connected to the additional column conductor via a respective monitoring resistor. Following the selective reading of the n*m sensor elements, the monitoring resistors between the individual column conductors and the additional row conductor and between the individual row conductors and the additional column conductor are read. If it is not possible to read one of the monitoring resistors, the test system can conclude that a conductor has broken in the row conductor or column conductor leading to the monitoring resistor. The appropriate row or column can then be deactivated by the evaluation circuit, so that incorrect interpretation of the seat-occupation situation is prevented.
A drawback of this apparatus is that, when a conductor break is recognized, the entire column or row affected by this conductor has to be deactivated and is therefore no longer available for actual situation recognition. This impairs the resolution of the sensor device, however.